(1) Field of the Invention
The present invention relates to subscriber telephone electronic hold circuits and more particularly telephone electronic hold circuits which require an automatic time disconnect of the hold condition after a suitable delay period.
(2) Background Art
Since the development of telephone circuitry employing active components, i.e.: solid state devices etc., additional features have become easy to add to the telephone. One such feature has been the "hold" circuit. With the provision of such circuit the telephone call may be electronically latched onto an artificial load allowing the handset to be returned to hookswitch, until the call is picked up on extension phone or at the original phone, at which time the hold condition is dropped automatically.
Previous hold circuits frequently make use of expensive components such as latching relays and/or integrated circuits voltage comparators and related associated circuitry adding substantial cost to the telephone. Sometimes these circuits have poor sensitivity because of the compromise between trigger "ON" sensitivity and the ability to drop completely on high resistance extensions and in the presence of low central office battery voltage. Some hold circuit methods require the use of a micro-processor and related circuitry to sense the line voltage change with conversion to a digital signal. It is obvious that all such circuitry suffers to some degree from overcomplexity with a reduction in the attendant reliability stemming from the increased number of components required in such circuit design.
Many of the existing hold circuits also suffer from the lack of inclusion of an automatic delay or time-out facility. In the event for example that a call is placed on "hold" and an extension or the original phone is not again taken off hook within a predetermined time period, it is desirable that the hold condition should be dropped, to avoid an excessive charge against the telephone subscriber. Therefore, it is desirable to add automatic time delay or time-out circuitry where the possibility exists that the hold condition will not be terminated within a predetermined period of time; say for example, six minutes, in order that the hold circuitry may be dropped automatically. There have been several means of providing such time delay functions. These have included for example, the use of integrated circuit voltage comparators sensing the charge on a capacitor and a microprocessor controlling the switching. While such circuits work they require relatively expensive support hardware and possibly the inclusion of additional software. Accordingly, it is the object of the present invention to provide a new and useful telephone hold circuit that includes an automatic disconnect feature.